matviar

I took my Cutlass convertible to the local car show hoping to win a trophy but instead bought a basket case 1970 Cutlass convertible. The non posi rearend is not worth putting back together for the cost, so I would like to find a posi with low 300 gear set. Let me know if anyone can help. I live in NW Indiana. Thanks Rick

djcrook73

I may be wrong but on the later gm's I know you can just change out the center section or chuck as some call it, swap the ring and pinion to desired ratio and bam! you now have limited slip. people save them so you may be able to find one and just rebuild the rear, as even if you bought another rear assembly from someone else, chances are that you would be smart to go through the whole thing (bearings, clutches, seals) anyway. not to mention shipping if not found locally. I was intimidated the first time, but it worked just fine with patience and research and making sure the fine tuning on the tooth contact pattern was done correctly.. I am by no means an expert on anything but best of luck with it. dave

djcrook73

I just re read your post and realized that you probably already knew that, but hope you find that rear for the 70

matviar

I have taken the rear end housing and a 300 posi series carrier to a shop and he said I would need a spacer ring to convert my 273 non posi to make the 300 series carrier to work. The mechanics and knowledge of this is beyond me.

Fun71

Are you trying to use a 671 carrier with the original gears (which by the way should be 2.78 for an O-Type 12 bolt; 2.73 are used in the 71-up 8.5" 10 bolt)? I don't think I have ever heard of someone trying to use a higher series carrier on a lower series ring gear - it's usually the other way around and that requires a ring gear spacer. Going the way I think you are saying would mean the carrier ring gear flange is thicker and the ring gear itself is also thicker, so I don't think it would work, and a ring gear spacer would make it even worse.



Below is some info on the O-Type posi and gear ratios that I got from Brian Trick many years ago:

O-Type 12 Bolt

Here is a list of the posi units & what gears they will fit:
#673 = 2.56 & 2.78 ratios, 28 spline
#671 = 3.08 & 3.23 ratios, 28 spline
#588 = 3.42 & anything numerically higher, 28 spline
#672 = 3.42 & anything numerically higher, 31 spline
***The 31 spline axles were offered in late 67-68 OAI cars only.

joe_padavano

If you are talking about swapping the carrier, that's how any rear axle is changed from open to posi/limited slip/anti-spin. The challenge is getting a posi carrier unit for a Type O.

matviar

The housing unit is original to the 1970 Cutlass convertible. It is a 12 bolt 273 gear open rear end. Yes I would need to use the spacer for using a posi carrier. I dont want to do that. I am looking for a GM 68-72 complete rear end wth posi and 323 gears on down.

joe_padavano

If it's the original rear end for that car, it is NOT a "12-bolt". It's a Type O axle used exclusively by Oldsmobile in the 1967-70 A-body cars. NOTHING on it interchanges with parts from any other GM axle except the brake backing plates. The number of bolts on the cover does not make it a "12-bolt". Search "Type O axle" and you'll find the details.

djcrook73

and that is why it is great to have guys like joe who really do know, as I learned something I would have never known and am the wiser for it.. thank god I gave the disclaimer that I was no expert ha ha.. great site for all who seek knowledge for sure!

matviar

Sorry, It is an an O type.

juiced71

the axle wont fit in an o rear ?

Fun71

No, not if you have the correct posi carrier. I posted the info above.